Fix GUI polish issues across auth modal, theme, and 404

- Add missing modal CSS (.modal-overlay/.modal-dialog/.modal-header):
  the "Connect to Microsoft" auth modal was rendering unstyled inline
  at the bottom of the page. Now a centered dialog with backdrop.
- Surface OAuth connect errors in the modal instead of silently
  reopening it with no explanation.
- MainLayout: implement IDisposable so event handlers are actually
  unsubscribed (Dispose existed but was never invoked).
- Wire up the Settings theme selector (was a dead control): drop the
  unsupported Dark option, call sptb.setTheme on save and on load,
  resolve System via prefers-color-scheme.
- Add branded 404 page via UseStatusCodePagesWithReExecute + Routes
  <NotFound> (blank white page before).
- Add .progress-fill.indeterminate animation and .progress-panel.
- Home: replace inline JS hover handlers with a .feature-card CSS class.
- Define missing --surface-2 variable referenced by MainLayout.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Components/Pages/Settings.razor

@@ -1,7 +1,9 @@
 @page "/settings"
 @attribute [Authorize]
 @inject IUserSessionService Session
+@inject IJSRuntime JS
 @rendermode InteractiveServer
+@using Microsoft.JSInterop
 
 <h1 class="page-title">Settings</h1>
 
@@ -19,7 +21,6 @@
         <select class="form-select" style="width:160px" @bind="_theme" @bind:after="Save">
             <option value="System">System</option>
             <option value="Light">Light</option>
-            <option value="Dark">Dark</option>
         </select>
     </div>
 </div>
@@ -44,14 +45,15 @@
     {
         var s = Session.Settings;
         _lang = s.Lang;
-        _theme = s.Theme;
+        _theme = s.Theme is "System" or "Light" ? s.Theme : "System";
         _autoTakeOwnership = s.AutoTakeOwnership;
     }
 
-    private void Save()
+    private async Task Save()
     {
         Session.UpdateSettings(new AppSettings { Lang = _lang, Theme = _theme, AutoTakeOwnership = _autoTakeOwnership });
         SharepointToolbox.Web.Localization.TranslationSource.Instance.SetCulture(_lang);
+        await JS.InvokeVoidAsync("sptb.setTheme", _theme);
         _saved = true;
         StateHasChanged();
         _ = Task.Delay(2000).ContinueWith(_ => { _saved = false; InvokeAsync(StateHasChanged); });